1. Field of the Invention
The present invention relates to doping of semiconductor III-V compounds with silicon and more particularly, to a method of silicon diffusion by thermal oxidation of a silicon capping layer.
2. Description of the Prior Art
In the fabrication of devices made from III-V compounds, such as gallium arsenide, doping is accomplished by ion implantation of the dopant elements. The control of the doping profiles in the layers of semiconductor material to obtain shallow junctions is generally achieved by implanting the dopants, such as silicon, to a predetermined depth as required to provide the desired electrical characteristics to the device. Subsequent to the step of ion implantation, it is necessary to anneal the semiconductor material to remove the damage caused by the implanted ions. Nevertheless, the damage that was produced by the ion implantation sets a limit on the electrical activation and carrier mobility in the III-V compounds. In addition, the activation efficiency typically degrades with an increasing dose of the implanted ions.
The diffusion of silicon into the III-V compounds has attracted attention as an alternative to ion implantation. Greiner et al. Appl. Phys. Lett. 44(8) 1984 disclose a method for diffusing silicon in gallium arsenide by using rapid thermal processing. The process includes depositing a layer of silicon by electron beam evaporation and subsequently depositing a layer of silicon dioxide by plasma enhanced chemical vapor deposition. Diffusion was accomplished by subsequently annealing the oxide capped silicon in a rapid thermal annealer, resulting in sheet resistances as low as 50 ohms per square and a maximum electron concentration of 5-6.times.10.sup.18 -cm.sup.-3. No measurable diffusion was obtained by annealing uncapped layers of silicon. The use of the two different processes for depositing the silicon and for depositing the silicon dioxide cap layer presents manufacturing difficulties and may cause deficiencies in device characteristics.